1. Field of the Invention
The present invention relates to a technique for driving a power device of a semiconductor device, and more particularly, to a single power source drive circuit for a power device of a half-bridge structure.
2. Description of the Background Art
A single power source drive circuit for a power device of a half-bridge structure, as shown in FIG. 10, comprises two switching devices Q2P and Q1P which are connected in series between a high-voltage d.c. power source 1P and a ground GND, and a high-voltage side drive circuit (i.e., gate driver) 2P and a low-voltage side drive circuit (i.e., gate driver) 3P which respectively drive the switching devices Q2P and Q1P. The drive circuits 2P and 3P turn on and off the two switching devices Q2P and Q1P alternately.
In the conventional drive circuit for a power device, a low-voltage d.c. power source 4P whose one end is connected to the ground GND supplies a voltage VCC to the low-voltage side drive circuit 3P, and charges a capacitor CP through a diode DiP. A voltage from the capacitor CP is supplied to the high-voltage side drive circuit 2P. Normally, diodes D2P and D1P are connected in inverse parallel connection to the switching devices Q2P and Q1P which are connected in series between the high-voltage d.c. power source 1P and the ground GND. The diodes D2P and D1P are used for regeneration of induced electromotive force when an inductance L is loaded.
The operation principles of the conventional drive circuit which is shown in FIG. 10 are as follows. That is, (A) when the switching device Q1P is ON, the capacitor CP is charged up on a path L1 from the low-voltage d.c. power source 4P in which the diode DiP, the capacitor CP and the switching device Q1P are disposed or on a path L2 from the low-voltage d.c. power source 4P in which the diode DiP, the capacitor CP and the diode D1P are disposed, and (B) when the switching device Q2P is ON, since the diode DiP is reverse-biased, the capacitor CP floats, and therefore, the voltage which is charged up the capacitor CP is supplied to the switching device Q2P.
However, in the conventional drive circuit which is shown in FIG. 10, since the low-voltage side switching device Q1P and the low-voltage side diode D1P are in the charging paths L1 and L2, respectively, on which the capacitor CP is charged up, a charging voltage VC to the capacitor CP is within the following range: EQU VC=(VCC-VD-VCE) to (VCC-VD+VF)
where an ON-voltage to the low-voltage side switching device Q1P is VCE, a forward-direction voltage to the diode D1P is VF, a forward-direction voltage to the diode DiP is VD, and a voltage in the low-voltage d.c. power source 4P is vCC.
Thus, while the inductance L is loaded, there are a period in which the switching device Q1P is ON so that the switching device Q1P carries a current (path L1) and a period in which the diode D1P carries a current (path L2).
Normally, the ON-voltage VCE for the low-voltage side switching device Q1P is around 2V, and so is the forward-direction voltage VF to the diode D1P. Hence, where the forward-direction voltage VD to the diode DiP is about 1V, the charging voltage VC to the capacitor CP varies in the following range: EQU VC=(VCC-3V) to (VCC+1V)
In other words, when a current flows in the path L1 so that the capacitor CP is charged up, the charging voltage VC is 3V lower than the voltage VCC of the low-voltage d.c. power source.
In such a case, the range of the charging voltage VC is from 12V to 16V in a 15V-driven switching device (in which VCC=15V). During driving with 15V, a gate voltage in the high-voltage side switching device Q2P in the ON-state is a minimum of 3V. This level of variation in charging voltage VC is said to fall within a usable voltage range.
However, in a 5V-driven switching device (in which VCC=5V), the charging voltage VC ranges from 2V to 6V. The variation in charging voltage VC is too great to use this circuit. That is, during driving with 5V, the gate voltage in the high-voltage side switching device Q2P in the ON-state is 0.5V to 0.75V, and hence, as the charging voltage VC drops as much as 3V from the drive voltage of 5V, even though the switching device Q1P turns on, an ON-resistance becomes large so that a big loss is created at the switching device Q2P. Even worse than this, there is a possibility that even driving of the switching device Q2P will become uncontrollable.
Thus, there is an inherent problem in the conventional single power source drive circuit for a power device of a half-bridge structure that the charging voltage largely changes due to the low-voltage side switching device which is created in the charging path for charging the capacitor which supplies a voltage to the drive circuit of the high-voltage side switching device or due to the diode which is connected to the low-voltage side switching device in inverse parallel connection. Hence, the conventional circuit can not drive a 5V-driven switching device with a low-voltage d.c. power source voltage of 5V. Such a problem must be overcome considering the recent trend that a voltage for driving devices have become lower and lower.